Sulfur vulcanization is a well-known chemical process for converting natural rubber or other general purpose elastomers into more durable materials via the formation of crosslinks between individual polymer chains through addition of and reaction with certain vulcanizing agents (also known as “sulfur-containing curatives”). In conventional processes for the manufacture of durable vulcanized elastomeric articles, a sulfur-containing curative is mixed with an elastomeric compound to form a vulcanizable elastomeric formulation that includes the sulfur-containing curative. The vulcanizable elastomeric formulation is subjected to a number of processing steps such as for example mixing, extruding, calendering, shaping, forming and building into the shape(s) of a desired “green” (unvulcanized) article or article component (“article”). The article is then subjected to conditions necessary to vulcanize the elastomer and form a vulcanized elastomeric article.
Current industry practice has embraced polymeric sulfur as a preferred vulcanizing agent in many commercial sulfur vulcanization processes. For example, U.S. Pat. No. 4,238,470, the disclosure of which is incorporated herein by reference, describes the use of polymeric sulfur as a sulfur vulcanizing agent for a vulcanizable elastomeric composition. Polymeric sulfur is generally characterized by a high molecular weight, a long, helical molecular structure and insolubility in carbon disulfide and other strong solvents as well as in rubber, rubber compounds and elastomers. In a typical sulfur vulcanization process step, a vulcanizable elastomeric formulation containing polymeric sulfur is subjected to conditions in which the polymeric sulfur converts to cyclooctasulfur (S8), a sulfur allotrope that is soluble in elastomers and oils and which therefore dissolves into the elastomeric formulation wherein it can take part in the vulcanization reactions.
Because the conversion of polymeric sulfur to cyclooctasulfur is temperature dependent and the effects of time and temperature on the conversion are cumulative, great care must be taken to ensure that the processing steps prior to final shaping, building or assembling of the vulcanized article prior to actual vulcanization do not initiate the conversion prior to the actual vulcanization step. Such premature conversion could result in sulfur “bloom”, a known phenomenon highly detrimental to interply adhesion and other vulcanized article characteristics. Sulfur bloom is the result of diffusion of soluble cyclooctasulfur and subsequent crystallization of sulfur on the surface of an uncured article and occurs when cyclooctasulfur concentrations in the green vulcanizable elastomeric formulation exceed their solubility limit in the formulation at a given temperature. The presence of sulfur bloom on the surface on an uncured article component or ply is highly detrimental to tack and adhesion of that component to other components or plies. In order to avoid premature conversion to cyclooctasulfur and the risk of bloom in vulcanizable elastomeric formulations with polymeric sulfur vulcanizing agents, current commercial practice includes limiting extended processing times to temperatures below about 110° C. or more preferably 100° C., as even a small percentage of conversion of polymeric sulfur to cyclooctasulfur may push the concentration past the solubility limit and create the potential for bloom. The shearing actions present in (and frictional heat generated by) the mixing, extrusion, calendering, shaping, forming, or other processing operations therefore present demanding temperature control challenges to the article manufacturers.
Management of these challenges typically involves a delicate balance between productivity, throughput, processing speed and product cost on one hand and product performance and quality on the other. Limitations implemented to reduce risk of premature polymeric sulfur conversion and bloom slow the manufacturing speed and thus reduce manufacturer profitability. Conversely a vulcanizing agent with less premature conversion propensity (and accordingly higher thermal stability) would increase manufacturing speed and accordingly the number of units a plant can create and the manufacturer's profit. In addition to faster manufacturing speeds, if the conversion from polymeric to cyclooctasulfur at any given temperature in the manufacturing process could be reduced, then the compounder has greater flexibility to incorporate more sulfur into a vulcanizable composition thereby having greater potential to manufacture goods of even higher quality and durability.
The prior art has attempted to improve polymeric sulfur thermal stability and retard or resist sulfur bloom through use of various stabilizers or stabilization treatments, as described for example in the above-mentioned '470 patent as well as U.S. Pat. Nos. 2,460,365; 2,462,146 and 2,757,075. Despite all these efforts, a continuing need exists for sulfur vulcanizing agents with higher thermal stability that translates to improved throughput and efficiency for vulcanized article manufacturers while avoiding the risks and detriments of bloom.